What are the differences between dominant whites and sabinos? In terms of phenotype, they do not look different.
As I mentioned in the previous post, sabino was once a catch-all term for a group of patterns that had a pretty broad range of expression. The first use of the term in an English-language book on horse color that I have found was Reiner Geurts’ Hair Color of the Horse, published in 1977. (The original Dutch book was published four years earlier.) Although it appears that Geurts took the term from an earlier Dutch paper written by J. K. Wiersema, he does state that it is an American term. I suspect that by this he may have meant South American, since it is a Spanish word that does not appear to have been widely used in the United States until the late 20th century. This is how Geurts characterized sabinos:
[The term refers to] a kind of piebald with often indistinct and irregularly bordered white radiating from under the breast, the belly and the extremities, and thence spreading laterally up the ribs and sometimes to other parts of the body. The rest of the coat is often ticked with white, or roan.
In 2005, scientists identified the mutation responsible for one of the sabino patterns. In their paper, the authors’ criteria for sabino was consistent with Geurts’ definition:
Horses characterized as having a Sabino white spotting pattern had three of four of the following characteristics: (1) two or more white feet or legs, (2) blaze (white patch extending the length of the face), (3) jagged margins around white areas, and (4) spots or roaning in the midsection.
The identified pattern was named Sabino-1, because the researchers expected to add more patterns to the series. From the same paper:
Consequently, we anticipate future reports for other genes encoding different Sabino phenotypes (possibly SB2, SB3, and so on)… The variant of KIT described in this study can explain some but not all the phenotypes described as “Sabino.”
That last sentence proved to be true. The resulting test is useful for only a small percentage of horses with a sabino pattern. So what happened? Where are the additional sabino patterns that were predicted?
The simple answer is that many sabinos, like the two Paint Horses pictured above, have patterns that have not yet been formally identified. That is, the mutation that causes them has not yet been found, which is necessary for the development of a test. But it is also true that along the way, the approach to naming the different KIT mutations seems to have changed.
Around the same time that Sabino-1 was identified, researchers began looking at a suspected Dominant White family in the Franches-Montagnes breed. Just like other researchers in the past, they indicated that the mutation did not always produce a completely white horse. The paper on Sabino-1 noted that these horses fit the phenotype for sabino, but that the pattern of inheritance matched the one described in the original studies of Dominant White. In 2007, a paper was published identifying the Franches-Montagnes mutation along with three others. These were named White-1 through White-4. Because these particular mutations consistently produced white or near-white mutations, and because the patterns appeared to be homozygous lethal, it made sense to categorize them as Dominant White and not Sabino. It should be noted, however, that the original paper outlining those first four Dominant White families did anticipate the possibility that future dominant white (W) mutations might not necessarily be lethal. In fact, it was not clear if all four of the initial mutations were lethal. Nonetheless, some saw the traditional distinction between the non-lethal Sabino patterns and the homozygous lethal Dominant White as significant for determining the appropriate category for a given pattern.
This became important when some of the patterns identified in subsequent studies more closely resembled Sabino than White. This was particularly true of the fifth White family, which originated with the Thoroughbred stallion Puchilingui. Puchilingui had what would have been called a “sabino roan” pattern, but his offspring were often loud, patchy sabinos.
Sato, pictured at the left, has what many would consider a typical pattern for a horse with the W5 mutation. It seemed that most horses that inherited this particular Dominant White mutation had a sabino phenotype. There were, however, some members of the family that could be described as white. This is the white Puchilingui daughter Shew.
With the identification of the W5 mutation, some began to refer to all highly-contrasted, patchy patterns as Dominant White while others (including many breeders) continued to prefer the term Sabino, even for those families with known mutations assigned a number in the W series. The confusion was increased with some of the later additions, particularly the two most recent Arabian mutations, W15 and W19, which did not seem to produce white phenotypes – or did so only very rarely. Would there be any new sabino patterns? Was the single incompletely dominant pattern likely to remain the first and only named Sabino pattern?
This question came up again when it was discovered that the reason some of the Puchilingui horses were white was that they carried two different KIT mutations. In addition to the already-named W5, white horses like Shew had another mutation that was initially overlooked because it was not thought to significantly alter pigmentation. It would appear that on its own, the W5 mutation produced what would have been called Sabino. Loudly marked, but still Sabino. It took adding a “quieter” pattern to produce the white or nearly-white phenotype.
The additional mutation was said to be “common and widely distributed”, and to have a “subtle white-increasing effect”. Of the 145 horses in the study, 52 were found to carry this particular mutation. Although only a handful of tested horses have published photographs, those that have been identified appear to be minimally-marked sabinos of the “flashy white markings” variety. In assigning the name W20, the authors gave the following reasoning:
We termed the new KIT variants W18 – W20 to provide a simple and unambiguous nomenclature for future genetic testing applications.
Some have interpreted this decision to mean that the name Sabino has been abandoned. It is an understandable conclusion, if horses like these two are not going to pick up the numbering in the Sabino sequence.
Yet perhaps overlooked is that in that same paper, the term Dominant White was also dropped. The previous seventeen mutations in the W series are referred to as Dominant White, but that phrase appears in scare quotes. Instead, the patterns are referred to as “White-spotted”. Sabino appears as a descriptive term (“sabino-like”) for one of the mutations described in the paper. The approach appears to be to number the new mutations to KIT consecutively in the W-series, regardless of the phenotype. That means that the “official” notation for the mutation may not correspond with the visual appearance of the horse. Given that diverse colors like tobiano and dark-headed roan have been found to share this same molecular location, it might be helpful to develop additional terminology that communicates visual differences that breeders find significant. From a molecular standpoint, dark-headed roan is one type of depigmentation, and is therefor a form of “white spotting” (and has been referred to in this way by scientists). A breeder interested in a predictable amount of white (either enough or not too much!), however, might have quite a different idea of what is and what is not white spotting.
It may be that phenotypes that fall clearly into existing groups, like roan (Rn), will retain their unique names. Unfortunately the appropriate category for many of the resulting colors – including some of the roan phenotypes as well as those factors that produce “ordinary” white markings – is likely to be less clear. While adding to the list of W mutations might provide “clear and unambiguous nomenclature” for tests, breeders and owners may find that they need additional names and categories to effectively communicate with one another.